In order to facilitate rapid detection of airborne pathogens that are capable of causing either natural or deliberate epidemics, it is of utmost importance to collect particles containing biological material directly from air in a form suitable for further analysis. Typically, particles containing or consisting of one or more biological organisms (bioparticles), are captured by passing (samples of) air through porous filters. To size fractionate for selected ranges of particles, a succession of filters have been used to select for the right size particles, which are subsequently collected at—and can be cultured on growth media plates within the collecting device (The Andersen sampler).
When bioparticles contain live organisms or spores, they can give rise to growth (colonies) on the media plate, which subsequently can be collected and analyzed easily. However, the researcher will have to wait for the colonies (colony forming units or CFU) to become visible, which, depending on the cultured organism, can take from days to weeks. Additionally, the possibility exists that a biological organism of interest might not thrive on the growth media. It is also possible that particles of interest are captured within or adhere to the filtering system and therefore go un-detected.
To compensate for sample loss a large volume of air need to be processed through the system. A complimentary method, which can be used in the combination with filter sampling, is to maximize the number of particles in a given volume of air, by using e.g. cyclones or other vortex type gaseous samplers that facilitate an initial concentration of particles. The later technique has also been used to concentrate particles into a volume of liquid, giving immediate access to captured bioparticles for a variety of different microbiological, biochemical, and molecular analyses.
Funnelling particles into a liquid is associated with heating and evaporation of the liquid and often volumes of liquid (larger than 1 ml) are required to prevent the system from drying out during the capture process. Subsequently, the liquid can be processed by centrifugation to perform a final concentrate of the sample.
Sampling of bioparticles can be done by air to air, air to surface or air to liquid methods. Liquid methods can be cumbersome because of freezing effects at temperatures below 0° C. Key aspects of the sampling are sampling volume, capture efficiency, and the concentrating efficiency of the sampling technology. A standard cyclone is capable of taking in one m3 of air per second and concentrating captured particles in 1-2 ml of fluid, which is a large volume in biochemical analyses. A volume in this range would fill a 96 well plate and consume reagents equivalent to approximately C 14 per plate per second. It is obvious that the rapid air sampling by the cyclone quickly is compromised by the subsequent sample preparation.
In one approach, electrostatic precipitation of airborne biological particles was utilized for sampling of bacteria onto agar plates in conventional macro scale devices (Mainelis et al 2002a; Mainelis et al 2002b) and for decontamination of a dental practice (Iversen & Tolo 1975).